1. Field of the Invention
The present invention relates to a novel method of and a novel apparatus for the production of steel pipes which are capable of making a steel pipe of excellent weld seaming and surface texture qualities with high productivity and which are adaptable to produce a wide variety of small product lots.
2. Description of the Related Art
Welded steel pipes are produced by subjecting a steel plate or a steel strip to cylindrical shaping and then to seam welding. Such steel pipe products of from small to large diameters have been produced by various methods among which electric resistance welding, forge welding or electric arc welding are typified.
In the production of steel pipes having small and medium diameters, electric resistance welding method has become predominant. This welding method is devised to cylindrically form a steel strip by a forming roll into an open pipe which is then heated at its two opposite lengthwise end faces by means of high-frequency induction heating at a temperature above the melting point of the steel strip. Those opposed end faces of the open pipe are subsequently butt-welded by a squeeze roll, whereby a steel pipe is obtained.
When an electric resistance weld pipe is produced by the above method of high-frequency induction heating, the open pipe is heated at its two opposite longitudinal end faces at a temperature higher than the melting point of the starting steel strip. Molten steel flows under the electromagnetic force, making a resulting oxide that gets into the butt-welded seam. This has a tendency to cause weld defects or molten steel splashes.
In order to overcome those problems, a method of producing an electric resistance weld pipe has been proposed in Japanese Unexamined Patent Publication No. 2-299782. Two different heaters are employed. A first heater is intended to heat two opposed edges of an open pipe at a temperature higher than the Curie point, and the second heater heats those opposed edges additionally to a temperature higher than the melting point of the starting steel strip and thereafter the open pipe edges are butt-welded by a squeeze roll. Japanese Unexamined Patent Publication No. 2-299783 also proposes a method of producing an electric resistance weld pipe employing two different heaters. Two opposite longitudinal edges of an open pipe are preheated with a current of a 45 to 250 kHz frequency applied in a first heater, and the open pipe edges are further heated at a temperature higher than the melting point of the starting steel strip in a second heater and thereafter butt-welded by a squeeze roll.
The foregoing methods of producing electric resistance weld pipes teach heating two opposite edge portions of an open pipe in uniform manner, but suffer from beads arising from flow of molten steel onto inner and outer surfaces of the pipe during butt welding since the open pipe edges are heated at a temperature higher than the melting point of the starting steel strip. The beads must be removed after butt welding. This removal is conducted laboriously by use of a bead-cutting tool.
However, such prior art methods encounter serious problems.
(1) The bead-cutting tool needs to be adjusted depending upon the amount of beads to be cut, with eventual losses of material and time.
(2) Due to its single-use character, the bead-cutting tool requires replacement, which entails costly shutdown of the associated production line.
(3) Especially in the case of steel pipe production at a high speed exceeding 100 m/min, the bead-cutting tool causes shortened life, ultimately involving frequent replacement.
Consequently, bead cutting imposes a bottleneck on production of steel pipes and prevents higher productivity.
Those production methods have another serious problem in that they fail to cope with production of small lots in a variety of steel pipe products. Such methods should choose those rolls meeting with the users' dimensional requirements, or otherwise should provide for an enormously increasing number of rolls of varied sizes.
With the above problems in view, it has been proposed to stretch reduce-roll an electric resistance weld pipe in a cold state, as described in Japanese Unexamined Patent Publication Nos. 63-33105 and 2-187214. Such stretch reduce rolling in a cold condition, however, invites a sharp rise in rolling load with eventual need for scaling up of a mill. It also requires a lubrication rolling unit to prevent seizing of the steel pipe which would result from contact of the latter with a roll. This results in added equipment cost and increased floor space in an uneconomical way.
In Japanese Unexamined Patent Publication No. 60-15082 and Japanese Examined Patent Publication No. 2-24606, there are taught methods in which an electric resistance weld pipe is stretch reduce-rolled in a hot condition. These methods have the drawback that because of exposure to reheating at from 800 to 900.degree. C. or higher, the steel pipe suffers from adverse scaling or scale biting during stretch reduce-rolling.
On the other hand, a highly productive method of making a forge-welded steel pipe is known to be suited for the formation of a steel pipe of a relatively small diameter. In this method, a continuously supplied steel strip is heated at about 1,300.degree. C. in a furnace and then shaped cylindrically by a forming roll to form an open pipe. High-pressure air is thereafter blown onto two opposed edge faces of the open pipe so as to remove scales, followed by blowing of oxygen onto the edge faces using a welding horn and by subsequent heating of the thus treated edge faces with the resultant oxidized heat at about 1,400.degree. C. Thereafter, these edge faces are butted by a forging roll and welded together at a solid phase, whereby a steel pipe is obtained.
The method discussed above, however, is deficient in the following respects.
(1) Because of failure of the open pipe to sufficiently remove scales at the edge faces, the scales bite into the forge-welded joint. This renders the welded joint inferior in strength to the mating matrix portion. Flattening tests have shown that the forged-welded steel pipe is extremely small in respect of its flat height ratio, say as low as h/D=0.5, as against the ratio or h/D=2t/D (t: steel strip thickness) attainable in the case of the electric resistance weld pipe.
(2) On account of exposure of the open pipe to heating at elevated temperature, scaling takes place on the pipe surface, bringing about a marred surface texture.
Despite its high efficiency with a pipe rolling speed of 300 m/min or above, the method in question leads to insufficient weld seaming and texture properties, eventually failing to produce a steel pipe having the strength and surface requirements satisfied as stipulated by STK and other provisions of JIS (Japanese Industrial Standards).